East View Cartographic: http://www.cartographic.com/
"Browse, buy and search online from more than 7,000,000 worldwide maps, geospatial data products, and geographic locations." Choices include: topographic, Digital Elevation Models (DEM), vector, imagery, atlases/books, geological/thematic, and nautical.
GIS Digital Spatial Data Clearinghouse: http://www4.uwm.edu/libraries/AGSL/agsgis/gislksA-Jdsd.cfm
University of Wisconsin-Milwaukee
Index Maps: http://www.lib.berkeley.edu/EART/ams.html
Index maps of maps put out by the U.S. Army Map Service (AMS) and the Great Britain, War Office, General Staff: Geographical Section (GSGS). Order numbers of the maps highlighted on the index maps are given. It is not clear whether these maps are still available for ordering.
MapRecord Publications: http://www.maprecord.com/
A reference work describing antique maps and listing their values.
National Geographic Magazine Map Collection: http://www.ngmapcollection.com/
Contains maps which accompanied the National Geographic Magazine for the last 100 years. Maps can be seen but this collection is designed to be used as a source for ordering copies of these maps. Many are topical.
The Royal Geographical Society: http://www.rgs.org/HomePage.htm (London) The image library includes maps which are both historical and modern and can be purchased.
USGS Topographic Maps http://topomaps.usgs.gov/
7.5 minute quadrangles for sale
Creating and printing maps through American FactFinder
Go to the Maps and Geography portion of the website and click on the link for Creating and Using Maps tutorial that appears in the center of the page.